1. Technical Field
The present disclosure relates to circuitry to control a voice coil motor, such as during the parking procedure of the read and/or write head of the hard disk at power off.
2. Description of the Related Art
In the state of the art, it is known that the hard disks of computers and the like are provided with a spindle motor for rotating the hard disk itself and a voice coil motor for moving the reading and writing heads.
In the event of a sudden lack of supply voltage, a procedure is implemented to first park the reading and writing heads, and then stop the spindle motor once the heads have reached the parking position.
In the absence of supply voltage, the spindle motor serves as a voltage generator, the voltage of which depends on rotational speed and its electric constant.
FIG. 1 shows the block diagram of a typical power combo used in applications for hard disks. The power combo is used to properly drive the spindle motor 2 and the voice coil motor 3 by a single driving circuit 1 which properly drives the power stage 12 for the spindle motor 2 and the power stage 13 for the voice coil motor 3, as better shown in FIG. 2. The driving circuit 1 comprises the driving circuit 10 of the power stage 12 of the spindle motor 2 and the driving circuit 11 of the power stage 13 of the voice coil motor 3. The power stage 12 comprises three pairs of transistors M1-M2, M3-M4, M5-M6 with respective pairs of diodes D1-D2, D3-D4, D5-D6 connected in parallel with each other between supply voltage Vmotor and ground GND, whereas the power stage 13 comprises two pairs of transistors M7-M8, M9-M10 with respective pairs of diodes D7-D8, D9-D10 connected in parallel with each other between supply voltage Vmotor and ground GND. The spindle motor 2 is coupled to the shared terminals of the pairs of transistors M1-M2, M3-M4, M5-M6, whereas the voice coil motor is coupled to the shared terminals of the pairs of transistors M7-M8, M9-M10.
The external supply voltage VCV feeding the power part may vary according to the type of application; in high end and desk top application it is typically of 12 volt while in mobile application it is typically of 5V.
The power combo, in addition to integrating the control of the two motors (spindle and voice coil motors), may comprise devices to implement other functions, e.g., voltage regulators and power monitor 4, serial port 5 and ISO-Fet 6.
The ISO-Fet is an internal element of the power combo that serves to insulate the internal supply line Vmotor from the external supply line VCV if the latter were to fail.
The ISO-Fet power up comprises a transistor connected to the voltage VCV and controlled by signal P; said signal P is adapted to shut down the transistor of the ISO-Fet when the voltage VCV falls under the threshold of the signal P, whereas it is adapted to keep it on when the voltage VCV is positive, that is above the threshold of the signal P.
When the VCV fails, the backelectromotive force voltage of the rotating spindle motor, i.e., the BEMF (Backelectromotive Force), is rectified to keep the internal supply line Vmotor at a potential enough to supply the section of the voice coil motor 3 for parking the heads.
The rectification of the backelectromotive force of the rotating spindle motor may be carried out by means of one of the following procedures, e.g., a passive rectification, a synchronous rectification of the BEMF of the spindle motor or a step up of the spindle motor.
The passive rectification implies a rectification of the BEMF of the spindle motor through the intrinsic diodes of the power stage 12 which is operated at high impedance.
The synchronous rectification of the BEMF of the spindle motor takes place in an active manner through the sequential power up of two MOSFET transistors of the power stage 12 in synchronicity with the phase of the three backelectromotive forces of the coils L1-L3 of spindle motor 2.
The rectification by means of the spindle motor step up implies that the power stage 12 is continuously switched from a tristate condition to a braking condition at a frequency higher than 20 KHz (out of the audible range), instead of being kept under the tristate condition. Thereby, when the power stage 12 is under the braking condition (with all the low side transistors being switched on or all the high side transistor being switched on), the spindle motor 2 is under a short-circuit condition and therefore the three backelectromotive forces are able to generate a current in the motor. When the power stage 12 is driven in tristate, the three motor currents generated during the braking step recirculate through the intrinsic diodes of the six transistors of power stage 12, thus loading the capacitance C3 connected between the line where there is the voltage Vmotor and ground GND, keeping it at an enough potential so as to supply the power stage 13 and voice coil motor 3 for parking the reading and writing heads; said parking procedure begins when rectifying the BEMF of the spindle motor 2.
The parking procedure of the reading and writing heads may be commonly carried out either at constant voltage or constant speed.
In the case of constant-voltage parking, the voice coil motor 3 is driven by the stage 13 applying a constant voltage for a certain time period T1, preset with an appropriate polarity for moving the heads in the correct parking direction, or the voice coil motor 3 is driven by the power stage 13 applying a first constant voltage for a time period T1 and a second constant voltage higher than the first voltage for another time period T2.
In the case of constant-speed parking, the voice coil motor is driven so as to keep the speed of reading and writing heads controlled during the parking procedure. Various methods are known in the state of the art to keep under control the speed by which the voice coil motor takes the reading and writing heads to a parking position. This type of procedure ends when the heads reach the parking zone; the control circuit 1 also comprises means adapted to detect when the reading and/or writing heads reach the parking position.
The control circuit of the parking procedure is supplied by the supply line Vmotor. However, for example, in mobile applications, the supply line Vmotor may not be high enough for correctly supplying the control circuit of the parking procedure. For this reason, an internal voltage regulator 20 supplied by a charge pump circuit 21 in turn supplied by the supply line Vmotor provides the supply voltage to the control circuit of the parking procedure. Therefore, in the case wherein the external supply voltage VCV is null, the charge pump circuit 21 provides to the internal voltage regulator 20 a supply voltage Vcp such as to correctly supply the internal voltage regulator 20.